Circuit testing may involve contacting a circuit with a probe. Nano-probe fault isolation systems for circuit testing use motorized nano-probes that can electrically contact a circuit and inject or detect signals. Such techniques are often used in the semiconductor industry. Nano-probing allows investigation of the electrical parameters of a nano-scale device. Probe systems may include a microscope, such as an optical microscope or scanning electron microscope (SEM), that provides an enlarged image of the probe and the work piece to facilitate the placement of the probe onto the desired region of the work piece. If an SEM is used, the probing is performed in the vacuum chamber of the SEM.
A critical step in the nano-probing process is probe landing, that is, the process of lowering the probe toward its respective target areas on the work piece until contact is achieved. Due to the minute dimensions of the nano-probe, this process is highly sensitive. Probe tips typically have a width of only a few tens of nanometers, and are easily damaged. As a result, probe touchdown event must be detected with extreme accuracy, preferably with an error of less than 50 nm. Continued forcing of the probe downward after touchdown can damage the probe, the work piece, or both.
Probe touchdown is currently detected manually by a human operator having significant operator expertise. The probe is navigated to the desired target area, while remaining above the work piece. The probe is then driven downwards while being monitored by the operator, who stops the downward motion when he detects contact between the probe and the work piece. The indication of probe touchdown is subtle, often seen as a shadow effect or slight darkening of the area around the probe tip in SEM images of the target area and surroundings.
Due to the subtleness and difficulty with detecting probe touchdown, the process is prone to damage both the probe and the work piece under investigation. In addition, the probes are expensive and manual probe touchdown provides limited probe longevity, requiring costly replacement.
A system for repeatedly and reliably lowering a probe to contact a surface is therefore desired, particularly an automated system that can be readily automated.